Dumbbell



FIG. 1 is a perspective view of a dumbbell showing my new design;

FIG. 2 is a front elevational view thereof;

FIG. 3 is a left side elevational view thereof, the side opposite being a mirror image;

FIG. 4 is top plan view thereof; and,

FIG. 5 is a bottom plan view thereof. 

The ornamental design for a dumbbell, as shown and described. 